1. Field of the Invention
The present invention relates to an apparatus for controlling a stepping motor used in feeding a sheet, for example in facsimile devices, etc.
2. Related Background Art
In a sheet feeding portion for vertical scanning (feeding) in a reading system or a recording system of a facsimile device, a stepping motor is normally used as a feeding motor, because the synchronous operation and the control of the sheet feed amount are easy to attain.
(I) The following two methods are conventionally employed in order to stop the stepping motor after it is in a driving state.
(1) The stepping motor is gently brought into a stop state from the driving state by controlling the deceleration of rotation in order to avoid out-of-step of the stepping motor. PA1 (2) Phase excitation upon stopping is kept on without a special control so as to stop the motor.
(II) Also, the conventional stepping motor is driven normally by the one-two-phase-on drive.
Supposing a facsimile device has a reading system for reading one line during horizontal scanning for a constant time a in reading storage area of a memory, and the drive speed of the motor changes depending upon the density of reading in the vertical scanning direction. A speed ratio of the motor is 4:1 between in the standard mode and the super-fine mode. In terms of the performance of the motor and the reading speed of one line, the stepping motor is driven so that one step of one-two-phase-on drive of stepping motor corresponds to the minimum feed amount in the vertical scanning direction (1/maximum vertical scanning density).
Recently, the reading speed of one line has been increasing and the speed of sheet feeding is also increasing therewith. Since the reading system of a facsimile device always requires accuracy of sheet feed in minimum feed amounts in the vertical scanning direction, the correspondence is maintained between one step of one-two-phase-on drive and the minimum feed amount in the vertical scanning direction. As a result, the drive speed of the motor has increased. The stepping motor, however, has the characteristic of decreasing its drive torque during high-speed driving. To compensate for this, a system for high-speed drive having a sufficiently high performance or a stepping motor increased in excitation current so as to increase the drive torque must be provided, to achieve an effective high-speed drive.
In the case of above (I), however, when the motor is stopped during high-speed rotation, the phase excitation upon stop is kept on so as to stop the motor, which sometimes causes noise, such as gear tooth noise in the sheet feeding system. Although the occurrence of noise can be prevented by gently stopping the motor by controlling its deceleration from the driving state, there is a drawback that the motor further moves from a position where it is desired to stop, in addition to a need for a complicated control mechanism, and the control of deceleration is not possible under all circumstances.
In the case of above (II), a motor which is selected to have a characteristic of excellent drive torque during high-speed drive, is naturally expensive, which is disadvantageous in respect of costs. The drive with an increased excitation current raises problems such as the generation of the vibration or heat of motor, an increase in load on a driver IC for driving the motor, etc. To solve this problem, it is conceivable that a control of current value is carried out in such a manner that the drive current to the motor is decreased during low-speed drive. However, the current during high-speed drive also increases and there are a lot of restrictions on the current control, because normal driver ICs have four settings of strong/medium/weak/off for the current values. For a control with finer current values, the control and circuits for the control become complex, presenting a drawback that the costs further increase.